The present invention relates to layered heat exchangers useful as evaporators for motor vehicle air conditioners.
Already known as such layered beat exchangers are two types; those having headers at one of the upper and lower sides of an assembly of plates in layers, and those having headers at these sides, respectively. Those of the former type have a heat exchange portion which is greater than in the latter type and are therefore expected to exhibit improved performance.
Stated more specifically, layered heat exchangers having the headers at one side comprise pairs of generally rectangular adjacent plates, each of the plates being formed in one side thereof with a U-shaped channel recess and a pair of header recesses continuous respectively with one end and the other end of the channel recess and each having a fluid passing opening, the plates being joined together in layers with the corresponding recesses of the plates in each pair opposed to each other to thereby form juxtaposed flat tubes each having a U-shaped fluid channel, and front and rear headers communicating respectively with opposite ends of each flat tube for causing a fluid to flow through all the flat tubes and the headers.
However, the conventional layered heat exchanger having the headers at one side has the problem that when used as an evaporator for motor vehicle air conditioners, the refrigerant fails to flow smoothly along the turn portion of U-shaped channel recess of each plate and to achieve as high an efficiency as is expected. This is because if the plates are designed, for example, to produce a rectifying effect, the refrigerant flow pressure loss can be diminished, but a reduced heat transfer coefficient and therefore an impaired heat exchange efficiency will result, whereas if the plates are conversely adapted to give a mixing effect chiefly, the refrigerant flow pressure loss increases to an undesirable level despite an improved heat transfer coefficient. The refrigerant is then liable to stagnate or flow unevenly especially in the vicinity of U-shaped turn portion of the refrigerant channel of each flat tube, consequently permitting the evaporator to exhibit impaired performance.
Further with the conventional evaporator, the joint between the plates is made by point contact, which therefore entails the problem that it is difficult to ensure pressure resistant strength.